The invention relates to a method of preventing contact of oxygen with a metal melt during continuous casting, in which the metal melt flows into a casting chamber bounded by walls and leaves this chamber as a stream, and also an apparatus for implementing the method.
In continuous casting, metal melt accumulates in the casting chamber and has to be protected against reoxidation and its bath surface has to be protected against high radiative heat loss. In conventional continuous casting, the bath surface is covered with casting powder or with an oil for this purpose.
For the casting of thin strips, various casting processes in which the casting chamber is formed not by rigid walls, but of a wall which moves with the stream or a plurality of walls which move with the stream, for example using a caterpillar chain as described in EP-A-0 526 886 or a roll as described in EP-A-0 568 211 or EP-B-0 040 072 or contrarotating casting rolls as described in U.S. Pat. No. 4,987,949 or EP-B-0 430 841, are known. In these methods, it is not possible to protect the metal melt reliably against reoxidation or heat loss by means of a casting powder or oil as is usually the case for casting chambers or casting dies having rigid walls.
EP-B-0 430 841 discloses, in the case of a two-roll casting unit, protecting the bath surface against excessively high radiative heat loss and against reoxidation by provision of a covering hood. However, for this solution it has been found that severe wear occurs at the contact surfaces between covering hood and casting rolls both on the covering hood and on the casting rolls and that, as a result of thermal deformation of the components, the entry of air and thus of oxygen through gaps between the walls bounding the casting chamber cannot be prevented. This results in reoxidation of the melt with all its disadvantages.
To minimize the entry of air through the gap between covering hood and casting rolls, U.S. Pat. No. 4,987,949 and EP-A-0 714 716 propose blowing an inert gas, preferably nitrogen or argon, into a defined gap between covering hood and casting rolls and thus to produce a barrier against intrusion of air. However, this measure is not sufficient to completely prevent air from entering the casting chamber and thus reaching the bath surface, so that, on the one hand, metal oxides are still formed at the bath surface and these lead to defects in the interior of the metal strip. On the other hand, metal oxides are formed at the surface of the solid shell forming around the stream or oxygen diffuses into the outer layer of the metal strip and there forms inclusions which increases the susceptibility to cracks. Despite the feeding-in of inert gas, air entrained in the microroughness of the roll surface is carried into the casting chamber in the laminar sublayer of the flow boundary layer. This sublayer adheres in the microroughness of the roll surface and can be stripped off neither by contacting, sliding seals nor by non-contact seals.
The invention makes it possible to avoid these disadvantages and difficulties and has the object of providing a method of the type described at the outset and an apparatus for continuous casting by means of which contact of oxygen with a metal melt can be prevented, which method and apparatus completely prevent reoxidation even when considerable wear occurs at the gaps between the walls forming the casting chamber. In particular, it should also be possible to remove the laminar sublayer of the air layer carried with or adhering to walls forming the casting chamber.
This object is achieved, in a method of the type described at the outset, by oxygen which attempts to enter via any gaps between the walls and/or is adhering to the walls being, removed by suction.
Particularly efficient removal of the oxygen can be advantageously achieved by carrying out the removal by suction in a plurality of extraction stages arranged behind one another from outside to inside towards the casting chamber, with the removal by suction advantageously occurring at a pressure which decreases from stage to stage from outside to inside towards the casting chamber.
Here, according to a preferred embodiment, the suction pressure in the extraction stage closest to the casting chamber is set to below 50 mbar, preferably below 10 mbar.
To ensure pressure equalization with the casting chamber, an inert gas is advantageously allowed to flow against the wall bounding the casting chamber directly adjacent to the extraction zone closest to the casting chamber, with the pressure of the inert gas advantageously being at least 10 mbar, preferably more than 200 mbar, above the pressure of the adjacent extraction stage.
Preferably, the inert gas is blown against the wall in a plurality of inert gas stages arranged next to one another from outside to inside towards the casting chamber.
The inert gas is advantageously blown onto the wall at a velocity of at least 0.5 m/s, not more than 10 m/s, preferably at more than 2 m/s.
In a casting process in which at least one wall is moved relative to the casting chamber, new regions of this wall which are about to enter the casting chamber are, according to a preferred embodiment, freed of adhering oxygen by removal of the oxygen by suction before entry. Continuous casting is then advantageously carried out by a roll casting process, preferably by a two-roll casting process, i.e. roll casting processes using only one casting roll, as described, for example, in EP-B-0 040 072, are also possibilities for application of the method of the invention. Of course, the method of the invention can also be applied in the casting of a metal melt on any moving cooling body, for example a caterpillar chain as described in DE-A-36 02 594. It is sometimes also advantageous for casting dies having rigid walls, e.g. if the application of a casting powder is not possible or would be too complicated.
An apparatus by means of which contact of oxygen with a metal melt can be prevented during continuous casting, in which a casting chamber bounded by walls is filled with metal melt and a stream leaves the casting chamber through a casting gap of the casting chamber, is characterized in that at any gaps present between adjacent walls there is provided an extraction device for oxygen attempting to enter via the gap and/or adhering to the walls.
In an apparatus for the continuous casting of a metal strip, preferably a steel strip, having two contrarotating casting rolls with parallel roll axes and two side dams which together form a casting chamber for accommodating molten metal and having a covering hood which is located above the casting chamber and closes off the latter at the top, and also having a sealing device which prevents entry of air into the casting chamber along a gap formed by the covering hood and the rotating casting rolls, the object of the invention is advantageously achieved by the sealing device being formed by at least one suction chamber, located on the atmosphere side in the vicinity of the gap between the rotating casting rolls and the covering hood and extending parallel to the roll axis.
This sealing device is particularly effective when it is made up of a plurality of suction chambers arranged next to one another in the circumferential direction of the casting rolls. Here, it is advantageous for each suction chamber to be connected via a suction line to an associated suction pump or a stage of a multistage suction pump. According to an embodiment which is simple in terms of construction, the sealing device is configured as a sequential multichamber system. As a result of this measure, the suction pressure decreases from suction chamber to suction chamber in the direction of motion of the casting rolls. Appropriate matching of the number of suction chambers to the circumferential speed of the casting rolls makes it possible to achieve complete removal of the air carried along with the rolls.
According to an improved embodiment, the sealing device is located at a defined distance from the casting roll surface and the gap formed by the sealing device and the casting roll surface is sealed, at least at the entry and exit sides, by means of contact seals, preferably brush seals or rubber lip seals. In this way, the entry of air is largely limited to the air carried along with the boundary layer even before the first suction chamber.
According to a further embodiment, at least one of the suction chambers is additionally equipped with an inert gas purge.
The apparatus is improved by locating an inert gas feed facility between the covering hood and the extraction device, with this inert gas feed facility being configured as a reduced pressure chamber having an opening directed toward the casting rolls. The opening is advantageously configured as a nozzle which is directed obliquely at the casting roll surface and is at an angle to the adjacent suction chamber. By means of this measure, an inert gas layer close to the roll is applied to the casting roll and excellent protection against access of oxygen or air is thus produced. If an inert gas layer of a few millimetres in thickness is applied to the casting roll and use is made of an inert gas which has a density higher than that of air, it is not necessary for the covering hood to directly adjoin the inert gas feed line and the extraction device.